1. Field of the Invention
This invention relates generally to brake systems for wheeled motor vehicles, and more specifically it relates to a brake-by-wire system.
2. Background Information
A brake-by-wire system may comprise a pedal that depresses a master cylinder plunger to perform the service brake function for a motor vehicle. The service brake function is performed by selectively applying electrically boosted hydraulic brakes or regenerative brakes. Hydraulic brakes generally apply friction forces to the wheels to dissipate energy as heat. Regenerative brakes may take any of various forms, but are often considered to comprise a rotary electric machine load that is applied to one or more driven wheels. When a regenerative brake is applied, energy flows from the wheels to operate the electric machine as a generator that converts the energy into electricity. The electricity is often used to charge an energy storage device, such as a battery. Whether hydraulic or regenerative brakes are applied is determined by an electronic brake controller.
The ability to operate a motor vehicle is typically controlled by a key-operated switch, sometimes referred to as an ignition switch. Such switches may assume various forms, but perhaps the most common in present motor vehicles is a mechanical switch that is turned by a metal key. When a proper key is used, the switch may operated from an OFF position to additional positions, such as START, RUN (i.e. ON), and ACCESSORY, to deliver electric power to electric circuits and devices that perform certain functions related to vehicle starting and operation.
Certain electric circuits need to be energized for a brake-by-wire system to properly operate. Placing the energization of such circuits under the control of the ignition switch is desirable so that they do not needlessly, drain the battery when the ignition switch is OFF. However, turning the ignition switch off to de-energize those circuits may remove power from devices that when powered, aid in reducing the pedal effort required to apply the brakes via the master cylinder. Hence, brake pedal effort may increase significantly when the ignition switch is turned off.
Governmental regulation applicable to certain motor vehicles proposes to mandate certain consistency in brake pedal effort that is independent of whether the ignition switch is OFF or ON. In the case of a brake system that comprises only boosted hydraulic brakes, a vacuum reservoir is used to provide a vacuum reserve that allows the booster to aid the pedal effort for several brake applications after the ignition switch is turned off to stop the engine. In the case of certain brake-by-wire systems, the de-energization of associated electric circuits that occurs when the ignition switch is turned off, precludes continuance of the ability to aid the pedal effort that was present when the ignition switch was on.
A preliminary novelty search developed the following U.S. Pat. Nos.: 5,139,121; 5,752,748; 5,769,509; 5,800,025; 5,834,854; and 5,923,096.